Alerting: fallbackText added to Google Chat notifier (#21464)

Add fallbackText to Google Chat notifications. 
Definition of fallbackText: A plain-text description of the message's 
cards, used when the actual cards cannot be displayed, 
e.g. mobile notifications.

Fixes #21463

pkg/services/alerting/notifiers/googlechat.go

@@ -55,6 +55,7 @@ Structs used to build a custom Google Hangouts Chat message card.
 See: https://developers.google.com/hangouts/chat/reference/message-formats/cards
 */
 type outerStruct struct {
+	FallbackText string `json:"fallbackText"`
 	Cards        []card `json:"cards"`
 }
 
@@ -187,6 +188,7 @@ func (gcn *GoogleChatNotifier) Notify(evalContext *alerting.EvalContext) error {
 
 	// nest the required structs
 	res1D := &outerStruct{
+		FallbackText: evalContext.GetNotificationTitle(),
 		Cards: []card{
 			{
 				Header: header{